1. Field of the Invention
The present invention relates to digital signal receivers. More specifically, the present invention relates to a digital variable rate demodulator operating close to the Nyquist rate.
2. The Background
In a communication system, data is typically formatted onto a carrier signal and conveyed via a transmitter. Once the signal travels through some intervening medium, it is received and decoded by the receiver. In theory, the waveform of the data transmitted would remain unaltered during the transmission process. However, in practice, the waveform is distorted and corrupted by its passage through the electronic circuitry of the transmitter and receiver, as well as, through the intervening medium. Thus, the receiver serves the dual purpose of decoding and insuring that the received signal is free from corrupt signal data that has been introduced during the communication process.
By example, in a standard satellite communication system a data signal is created at one location, encoded onto a radio signal, and transmitted to a satellite which may be in synchronous orbit above the earth. The satellite then retransmits the received signal to another location, where it is received and demodulated. In such a communication process, the data carrying signal is likely to have passed through several electronic systems, free space distances in excess of 40,000 miles, and twice through the atmosphere; subjecting the signal to numerous encounters with interference and distortion.
Historically, the transmitted signal had a fixed data rate known to the receiver which allowed for the receiver to be configured for the precise characteristics of the known transmitted signal. Currently, the trend is toward having the capability to vary the data rate to accommodate the desire for variance in satellite channel signals. For example, a single satellite channel may be used to carry various types of data signals, some of which are transmitted at low data rates and some of which are transmitted at high data rates. In another example, when the satellite signal carries compressed data, it may be desirable to vary the data rate for a given channel depending on the programming to be broadcasted. Thus, based on the differences in the speed of the action, a video feed of public service announcements would require a lower data rate than a video feed of a more data-intense activity, such as, a sporting event.
Current variable rate digital communication receivers, such as satellite receivers or cable receivers, use hybrids of analog and digital techniques or pure digital circuitry to implement the filter scheme within the demodulator. A demodulator within a digital signal receiver acts to recover an original signal from a modulated carrier.
In order to provide a variable bandwidth filter structure, the analog/digital hybrid techniques rely on changing the sampling frequency and using different filters according to the required bandwidth. Thus, for a given data rate and given filter structure, the filter bandwidth can be adjusted by changing the sampling frequency. Ordinarily, this would be accomplished by use of a single digital match filter. This type of filtering scheme allows for the filter to be modified by changing the sampling frequency in order to filter variable bandwidth signals.
However, in using such a analog/digital hybrid scheme it is not viable to reduce the sampling rate below the level dictated by the front-end anti-aliasing filter. This level can also be defined as the Nyquist rate (twice the Nyquist frequency which is equivalent to the width of the band of frequencies within the waveform). In practice, when the sampling frequency falls below the Nyquist rate aliasing occurs. When aliasing occurs, frequencies greater than one-half the sampling rate become indistinguishable from frequencies in the fundamental bandwidth and the overall system is disrupted. For video communications this would result in unacceptable degradation to the picture and sound. To compensate for this anomaly at lower sampling rates, a lowpass filter with a lowpass bandwidth is required. This means that for a typical application numerous filters will be required to achieve the necessary output. These front end analog filters tend to be bulky and, thus take up critical area on the surface of an integrated circuit. In addition, the use of numerous filters in a given demodulator design readily becomes cost prohibitive in commercial applications where today's markets demand cost efficiency.
Alternatively, systems have employed the use of pure digital circuitry to implement the variable bandwidth filter structure. The digital variable bandwidth filter structure is typically constructed from decimation filters. Decimation is a means by which the digitized video waveform is image scaled. By using filters to accomplish the scaling, aliasing concerns are limited and image artifacts are smoothed over allowing for further signal processing to proceed error-free. Filter decimation is typically accomplished by bandwidth limiting the image horizontally and vertically. However, each scaling factor requires different filter coefficients and adds to the complexity of the filter. Thus, in these systems when the need arises to tune to lower frequency rates it becomes imperative to use filters with narrower bandwidths. As the bandwidth narrows in these filters the complexity of the filters increases, accordingly. This results in the need to implement filters which are large in size and require high power in operation. As is the case with a analog/digital hybrid scheme, use of a pure digital scheme presents the similar problems of area consumption on a given integrated circuit and, ultimately, cost efficiency. The cost of implementing large complex decimation filters readily becomes impractical in the current commercial integrated circuit market.